This invention relates to data-storage systems, and in particular, to the pre-fetching of data by such systems.
In a data storage system, data can be stored on a mass-storage device or in a semiconductor memory. Mass-storage devices, such as disks, offer considerable capacity at low cost per unit of data stored, but long access times. In contrast, semiconductor memory provides much shorter access times at higher costs per unit of data stored.
Many database applications perform queries that require retrieval of records from one or more mass-storage devices. Each retrieval of a record requires accessing the mass-storage device to copy data to a semiconductor memory. In processing such queries, it is desirable to pre-fetch records and store them in the semiconductor memory. This reduces latency because the database application can then process records already in memory while another process pre-fetches additional records.
Pre-fetching records is effective at reducing latency only to the extent that the pre-fetched records are actually sought by the database application. The success of a pre-fetch policy thus depends on the extent to which one can correctly anticipate the needs of a database application.
The invention is based on the recognition that in many cases, the same or virtually the same data is periodically accessed by an application process. As a result, it is possible to identify locations of data that will soon be sought by an executing application on the basis of locations of data already retrieved that application.
One practice of the invention includes monitoring first information representative of recent accesses to the storage medium by a currently-executing application and comparing this first information with second information representative of previous accesses to the storage medium by a previous execution of the same application. This comparison provides a basis for determining a likelihood that the application will require access to remaining data identified by the second information.
An additional step of causing data identified by the second information can be added. In some aspects of the invention, all remaining data identified by the second information is pre-fetched. To reduce the possibility of unnecessary pre-fetching, other aspects of the invention provide for causing only a limited portion of the data to be pre-fetched. In some practices of the invention, the extend of this limited portion is fixed, whereas in other practices, the extent is variable and depends on the likelihood that the application will require access to data identified by that information.
Another practice of the invention includes monitoring a first sequence of locations accessed by an application and then determining, on the basis of that first that the application will require access to locations in a second sequence of locations.
The invention is also embodied by one or more articles of manufacture on which are encoded instructions for carrying out the methods described above. The articles of manufacture include one or more machine-readable media on which are encoded machine-executable instructions that, when accessed, cause a machine to carry out the foregoing methods.
Other embodiments of the invention include data-storage systems having a first storage medium for long-term storage of data and a second storage medium for temporary storage of selected data stored on the first storage medium. There is also available to the data-storage system historical data containing information representative of an application""s past access to the data on the first storage medium.
Such data-storage systems include a snooping process that monitors current access by the application to data stored on the first storage medium and a matching process in communication with the snooping process for receiving therefrom information indicative of data accessed by the application. The matching process is configured to determine, on the basis of the received information, a likelihood that the process will access data identified by the historical data.
These and other features and advantages of the invention will be apparent from the following detailed description and the accompanying figures, in which: